The present invention relates to an oil quality sensor, to a process for oil quality determination, and to the use of a sensor.
In the course of time motor oils have developed into high-tech products, which are one of the prerequisites for ensuring high engine performances. In this connection the term "motor oils" is a collective designation for basic oil components made of mineral oil, hydrocracking agents and synthetic components. Motor oils also contain an additive which is added in the form of a ready-made mixture ("packet"), as well as viscosity improvers (VI). Motor oils serve as lubricants for the engines as well as cooling and sealing media. Furthermore, they are intended to clean and keep clean all engine parts. The VI improvers ensure a more favorable viscosity temperature behavior than is shown by the pure basic oils. Depending on the requirements, the proportion of additives and VI improvers added to the oil usually ranges between 5 and 25%.
Further tasks of the additives in motor oils are: improvement of the corrosion-protection properties of the oils and reduction of sludge deposits and oil thickenings, as well as abrasion protection at frictional partners under all occurring loads. The thermal stresses on motor oils are high: in the sump they average about 100 to 150.degree. C., and in the region of the upper piston ring zone, temperature peaks of between 200 and 350.degree. C. may occur.
During their use the oils age, and primarily it is the additive components and VI improvers that are decomposed (used up). Unused, partially oxidized and polymerized fuel components are responsible for a considerable part of oil aging. Thus, the aging of oil is brought about by the effect of temperature and reactive combustion products (radicals) as well as by exceeding the dispersibility of the oils for solids and products of aging. As a result, the properties of the oil necessary for trouble-free operation of the engines are drastically worsened. An increased viscosity has the effect, e.g. at start-up, of causing a more prolonged transport of the oil to the places to be lubricated, resulting in increased abrasion.
The consumption of dispersion additives results in worsening of the ability of the oils to keep the engines clean, particularly at critical lubrication points such as the region of piston rings/slots and top lands, and in worsening of the prevention of deposit formation at valves and in valve drive.
Hence it is desirable to continuously or at brief intervals determine the deterioration of the properties of mineral oils which necessarily occur during engine operation, i.e. to determine it e.g. one or more times during the operation of a combustion engine.
However, until now reliable sensors for oil condition analysis have not been successfully developed, so that for prolonged operation of a motor oil in the engine, particularly in nonstationary engines, it is necessary to carry out an on-board analysis, i.e. an analysis done directly on the engine.
So far various oil sensors have been developed which measure the viscosity, TAN (total acid number) or fill level, in particular. Here a special difficulty is due to the use of different oils in the same combustion engine, as well as the compensation for different aging effects on the measured property. For example, it is known from U.S. Pat. Nos. 4,675,662 and 4,721,874, EP No. 527,176 B and JPN. Appl. Phys. 1993, Acoustic Plate Viscosity Sensor, to utilize the aging-related change in viscosity of the oil as a parameter for the oil condition. This is done via acoustic transit-time changes, phase shift or via resonance frequency changes of a quartz oscillator. Here, problems are presented by the frequent lack of a possibility to perform the measurement on board, on the one hand, and by the possible counteracting effects of "decomposition of the motor oil and dilution by fuel" on the other hand, an effect which reduces viscosity, as opposed to the "linkage of decomposition products." which increases viscosity, as long as these products do not separate out as sludge.
Because of their basic principle, neither TAN or TBN (total basic number) is suitable for an on-board measurement, inasmuch, in these methods, old oil is titrated with KOH. More recent set-ups, e.g. those known from SAE 910497, SAE 962112, U.S. Pat. Nos. 4,675,662, 4,792,791 and 5,200,027 show interesting solutions which, operate e.g, with capacity sensors, measurement of ionic migration or a potential difference, with electrochemical solid cells or with corrosion sensors. Some of these set-ups are inexact, still too large and too heavy, or require a sacrificial structural component which is basically undesirable. Also, mathematical models (SAE 870403) and HC-waste gas sensors (DE 42 35 225) are known which so far did not lead to a breakthrough. Nor are fill level sensors very suitable, since they fail when e.g. the engine oil is greatly diluted by fuels.
In "Molecular Imprinting of Chemically Sensitive Coatings--New Strategies in Sensor Designs and Fabrication" by F. L. Dickert, P. Forth, P. Lieberzeit, M/ Tortschanoff, W.-E. Buist, U. Knauer and G. Fischerauer in "Sensor 97", 8. International Fair, Nuremberg, 1997, molecular imprinting for mass-sensitive sensors in gases and liquids is described in detail.